Photochromic plastic materials, particularly plastic materials for optical applications, have in recent years attracted growing commercial interest due to their light weight in comparison to glass. When exposed to light radiation containing ultraviolet (UV) rays, such as the ultraviolet radiation in sunlight or the light of a mercury lamp, photochromic compounds exhibit a reversible change in color. When the ultraviolet radiation is discontinued, the photochromic compounds return to their original color or colorless state. Ideal photochromic compounds for use in optical applications change color efficiently upon exposure to near ultraviolet light, resist bleaching in white light and have a relatively fast ability to fade at ambient temperature.
A variety of photochromic compounds have been proposed for use in applications in which a sunlight-induced reversible color change or darkening is desired. U.S. Pat. No. 3,567,605 to Becker describes a series of pyran derivatives, including certain benzopyrans and naphthopyrans. U.S. Pat. No. 4,563,458 describes chroman and chromene compounds intended for use in pharmaceutical compositions. European Patent Publication 246,114 and U.S. Pat. No. 4,826,977 describe a series of photochromic spiropyrans in which an adamantane group is attached at the position adjacent to the oxygen in the pyran ring. U.S. Pat. No. 4,818,096 and European Patent Publication 250,193 describe photoreactive plastic lenses that include the photochromic spiropyrans of European Patent Publication 246,114 in combination with a blue photochromic benzopyran or naphthopyran having an aminophenyl substituent at the position adjacent to the oxygen in the pyran ring. The '096 patent also discloses 2,2,8,8-tetraaryl-2H,8H-[1,2-b:5,6-b']dipyran, a compound based upon 1,5-dihydroxynaphthalene. European Patent Publication 294,056 describes a process for producing a polyurethane plastic having photochromic properties; the photochromic compounds contained in the plastic include, among others, a naphthopyran derivative in which the pyran ring is substituted at the position adjacent to the oxygen in the pyran ring with di(p-methoxyphenyl) substituents.
Padwa et al. in J. Org. Chem., Volume 40, No. 8, 1975, page 1142, describes the investigation of photochemical reactions of 2,2-dimethylbenzopyran and related compounds, and identifies fatigue products and suggests pathways to the ring-opened colored intermediates and the phenolic degradation product. The authors do not suggest ways in which the compounds might be improved, nor any modification that might be made to the structure of the known pyran compounds.
Inadequate fade rate has plagued--to a greater or lesser degree--certain of the prior art photochromic compounds. A need therefore remains for photochromic compounds which exhibit adequate fade characteristics yet preserve the other desired characteristics for a photochromic compound.